Water loss usually is the most important factor causing spoilage of one third of the world's fresh horticultural commodities during storage, transport and distribution. The limit to how much water can be lost before fresh plant matter becomes unsalable varies from approximately 3% for lettuce to 10% for cabbage and celery, for most commodities is between 5 to 7%, and weight loss from cut-flowers must be kept below 10% to avoid senescence and a large decrease in vase-life.
Conduction, convection, radiation, and evaporation or condensation modulate the vapor pressure and temperature gradients which develop in systems containing plant matter, and heat transferred by evaporative cooling determines the amount of water the plant matter loses. During storage in a refrigerated space water loss depends on respiratory heat, sometimes augmented or reduced by additional heat transferred to or from the plant matter by convection and radiation. Respiratory heat is immediately available since it is generated within plant matter and does not have to be acquired from the environment.
Water loss by evaporative cooling lowers a stored commodity's temperature unless the latent energy used to change the state of water from liquid to vapor is replaced from a heat source. When stored plant matter remains at a constant temperature if the heat necessary to evaporate the water transpired by the plant matter is less than the respiratory heat the plant matter must transfer heat to the environment, and if the heat used to evaporate transpired water exceeds the respiratory heat the plant matter is acquiring heat from the environment.
Hypobaric storage systems are precisely controlled combinations of low pressure, low temperature, high humidity and ventilation that vastly extend the length of time a perishable commodity remains fresh. Atmospheres are tailored to each perishable item.
A ‘metabolic humidification system’ operates during every hypobaric storage, evaporating the amount of water from plant matter into the storage atmosphere that is needed to continuously transfer most of the respiratory heat plus any additional heat which the plant matter is receiving from the environment. Convection is ineffective in transferring these heat sources because the convective process is 80 to 90% inhibited at a low storage pressure according to Equations 6 and 7, infra. In LP warehouses and laboratory systems the ‘metabolic humidifier’ is supplemented by a ‘mechanical humidifier’ (FIG. 1—A) which warms and evaporates supplementary water (FIG. 1—B) by means of electrical heat (FIG. 1—E) in order to saturate low-pressure air at the selected storage temperature and pressure before said air enters the hypobaric storage space. The advantage gained by using a mechanical humidifier is that a full load, partial load, even a single fruit, vegetable or cut-flower can be stored in a hypobaric warehouse or laboratory apparatus confident that the humidity will always be saturated to minimize commodity water loss.
Super-Saturation and Mist Formation in LP Chambers
At atmospheric pressure and 100% RH respiratory heat warms plant matter above the ambient air's temperature and creates a vapor pressure gradient between the plant matter and ambient air. When convection cools water vapor transpired across the vapor pressure gradient ‘heterogeneous’ mist formation sometimes occurs on cloud condensation nuclei (CCNs). The international definition of fog is a visibility of less than 1 km (3,300 ft.); mist is a visibility of between 1 km (0.62 miles) and 2 km (1.2 miles); haze extends from 2 to 5 km (1.2 to 3.1 miles). Water droplets have a radius of approximately 5.0 to 7.5 microns (μm) in dry fog; 10 to 15 μm in wet fog; and 30 μm in mist. There are approximately 100 to 1000 CCN nuclei per cc of atmospheric air, but within a hypobaric storage space the CCN frequency is decreased proportionate to the reduction in air partial pressure. Only 0.5 to 12 CCNs per cc are present at a storage pressure of 10 to 20 mm Hg, making heterogeneous condensation unlikely to occur. Homogeneous condensation occurs in air lacking CCNs, where droplet formation and growth depends on statistical collisions between water molecules. Homogeneous condensation cannot occur during LP storage because 400 to 600% super-saturation is needed for homogenous condensation of pure water vapor to begin.
Water vapor is likely to condense and form mist if water-saturated air is less than 2.5° C. warmer than a heat exchanger's surface and the air in the stagnant surface layer cools faster than water can be removed by mass transfer. See BROUWERS, H. J. H. (1990) An improved tangency condition for fog formation in cooler-condensers. Int. J. Heat Mass Transfer 34 (8): 2387-2394 (“Brouwers 1990); BROUWERS, H. J. H. (1992) Film models for transport phenomena with fog formation: the fog film model. Int. J. Heat Mass Transfer 35 (1): 13-38 (“Brouwers 1992a); BROUWERS, H. J. H. (1992) A film model for heat and mass transfer with fog formation. Chemical Engineering Science 47 (12): 3023-3036 (“Brouwers 1992b); BROUWERS, H. J. H. and CHESTERS, A. K. (1992) Film models for transport phenomena with fog formation: the classical film model. Int. J. Heat Mass Transfer 35 (1): 1-11 (“Brouwers and Chesters”); SALEH, R. (2004). Webfea-lb.fea.aub.edu.lb/proceedings/2004/SRC-ME-15.pdf (“Saleh, 2004”). A low air velocity, such as that occurring during LP storage, is one of the most important factors causing mist or fog to form on a heat exchanger's surface, and at a water vapor mass fraction between 0.05 and 0.45 condensation may occur and create fog in a stagnant wall film. See KOYAMA, S., YASHUHARA, K. and YARA, T. (2002) Study on mist formation from humid air cooled in a rectangular tube. Engineering Sciences Report Kyushu University 24 (2): 187-193 (“Koyama”); Brouwers, 1990, 1992a, 1992b; Brouwers and Chesters, 1992. During hypobaric storage mist formation is likely to occur in the stagnant gas layer beneath the storage space's roof because the flow is laminar, the temperature difference between plant matter and the wall is less than 2.5° C., and the vapor mass fraction in an empty chamber is 0.46 to 0.56. The vapor mass fraction is even higher when plant matter is present, and the roof is a refrigerated heat-exchange surface. Any mist or fog that forms during hypobaric storage is in equilibrium with the super-saturated low-pressure chamber atmosphere because the equilibrium vapor pressure over a convex mist or fog water droplet is higher than that over a plane surface:er=e∞exp[a/r]  Equation 1where er (atm.) is the saturation vapor pressure of a water droplet of radius r (μm), e∞ is the saturation vapor pressure over a plane surface of water (atm.), and a≈3.3×10−7/T (meters). The fraction er/e∞ increases with a decrease in droplet radius (Table 1). At a 10 μm radius er/e∞=1.0012 corresponds to 0.12% super-saturation. This is the degree of super-saturation required for a 10 μm radius wet fog water droplet to be in equilibrium with the surrounding water-vapor partial pressure. The droplet will evaporate if super-saturation is below the indicated value for the droplet radius, and increase in size if it is higher.
TABLE 1Fraction er/e∞ for different sizes of droplets at 0° C.r (μm)10−210−110100er/e∞1.1281.0121.00121.0001See TIEDKE, M. (1987) Parametrization of non-convective condensation processes. European Centre for Medium-Range Weather Forecasts. Lecture Series. 9 pps.
The heat transfer coefficient for condensation is greatly reduced when non-condensable air is present in the storage atmosphere, even in very small amounts. The air is left behind when water vapor condenses on a cold surface and the incoming condensable water-vapor must diffuse through the air-enriched mixture collected in the vicinity of the condensate surface before reaching and condensing in the stagnant film layer at the cold surface. The presence of non-condensable air adjacent to the condensate surface acts as a thermal resistance barrier to convective heat transfer, reducing the heat-transfer coefficient for condensation by at least an order of magnitude. See ÖZISIK, M. N. (1985) Heat Transfer. A basic approach. McGraw-Hill, New York (“Özisik”); KNUDSON, J. G., BELL, K. J., HOLT, A. D., HOTTEL, H. C., SAROFIM, A. F., STANDIFORD, F. C., STUFLBARG, D., and UHL, V. W. (1964). Heat transmission. In: Crawford, H. B. and Eckes, B. E. (ed). Perry's Chemical Engineer's Handbook. McGraw-Hill, New York, Section 10, pps. 1-68 (“Knudsen”). In a hypobaric system where the vapor mass fraction of water vapor often is larger than 0.5 this effect can allow a significant amount of super-saturated air to escape in the air-change.
Control of Pressure
Vacuum breakers control the pressure in a vacuum chamber by regulating the rate at which atmospheric air enters while the vacuum pump withdraws low-pressure air at a constant rate. Vacuum regulators (FIG. 1—H and FIG. 2—N) maintain a constant process pressure at their inlet by throttling flow from their outlet to the vacuum pump (FIG. 1—N and FIG. 2—P). Breakers and regulators used in hypobaric storage systems are referenced to an absolute total vacuum to eliminate errors caused by fluctuations in a barometric reference pressure, and are able to control the pressure ±0.2 mm HgA.
The pressure in a VivaFresh hypobaric warehouse was measured with a Honeywell ASCX15AN absolute pressure transducer, and controlled by three Clippard EVP proportional solenoid valves acting as an absolute vacuum breaker in response to a proportional integral derivative computer-controlled algorithm. See EP20100267144. The EVP system maintained the pressure ±0.2 mm Hg and had sufficient capacity to serve as a vacuum breaker in both hypobaric warehouses and VacuFresh hypobaric intermodal containers. However, the EVP system's flow capacity is many hundred-fold too small for use as a vacuum regulator in a hypobaric warehouse or VacuFresh intermodal container.
Pressure in hypobaric storage systems also has been controlled by manually balancing needle valves which adjust the inflow of humidified air into a vacuum chamber vs. the rate at which the chamber air/water-vapor mixture is evacuated by the vacuum pump. A static pressure regulating system employed in many Chinese hypobaric storage laboratory systems halts evacuation after the desired pressure has been reached, and intermittently resumes pumping to return the chamber to the set pressure after air has been intentionally reintroduced or has leaked into the chamber. These methods are not referenced to an absolute pressure and are imprecise.
Storage Boxes
Heat transfer (Qv) by water vapor evaporation through the storage box's walls and vent holes to the atmosphere is:
                                          Q            .                    v                =                                            m              v                        ⁢                          H              v                                =                                    [                                                                    A                    ⁡                                          (                                              Δ                        ⁢                                                                                                  ⁢                                                  p                          v                                                                    )                                                        ⁢                                      H                    v                                                                    r                                      v                    ,                    box                                                              ]                        ⁡                          [                                                M                  v                                                                      R                    u                                    ⁢                  T                                            ]                                                          Equation        ⁢                                  ⁢        3            where mv (kg) is the weight of water (v) evaporated, Hv (kcal/kg) is the latent heat of water evaporation [595.4 kcal/kg@0° C.; 590.2 kcal/kg@10° C.; 584.9 kcal/kg@20° C.], A (m2) is the box's surface area, Mv is the molecular weight of water (=18), T is the temperature (K), rv,box is the box's resistance to water vapor transfer (s/m), ΔPv is the water vapor-pressure gradient between the box and chamber air (atm), and the gas constant Ru equals 0.08295 m3·atm/kg·mol·K. The box's transpirational resistance (rbox) depends on the storage pressure and water-vapor pressure according to Equation 4:
                              r          box                =                              r                          box              ,              R                                ⁢                                    ln              ⁡                              [                                                                            p                      R                                        -                                          p                                              V                        ,                        O                                                                                                                        p                      R                                        -                                          p                                              V                        ,                        i                                                                                            ]                                                    ln              ⁡                              [                                                      p                    -                                          p                                              V                        ,                        O                                                                                                  p                    -                                          p                                              V                        ,                        i                                                                                            ]                                                                        Equation        ⁢                                  ⁢        4            where rbox=rv,box (Equation 3) and rbox,R is the box resistance measured at reference pressure PR (atm) for vapor pressure values pV,i and pV,O inside (i) and outside (o) the box, and p (atm) is the storage pressure. See BURG, S. P. and KOSSON, R. L. (1983) Metabolism, heat transfer and water loss under hypobaric conditions. Lieberman, M. (ed.) Postharvest Physiology and Crop Preservation Plenum Press, New York, pps. 399-424 (“Burg and Kosson, 1983).
The humidity in VacuFresh containers decreases by approximately 50% during a 2-3 week period, and continues to decline because plant matter progressively produces less respiratory heat and therefore transpires less water-vapor. See HARDENBURG, R. E., WATADA, A. E., and WANG, C. L. (1986) The Commercial Storage of Fruits and Vegetables, and Florist and Nursery Stocks. U.S.D.A. Dept. of Agric. Handbook No. 66 (revised) (“Hardenburg”); BURG, S. P. (2004) Postharvest Physiology and Hypobaric Storage of Fresh Produce. CAB International, Wallingford, Oxfordshire, UK, 654 pps (“Burg, 2004”); FIG. 4. The reduction in chamber humidity accelerates evapo-transpiration from the plant matter, causing said matter's temperature to approach the dew-point temperature of low-pressure air in the storage space. Heat then transfers by radiation from the warmer chamber walls to cooler exterior boxes, and to a lesser extent by convection from the warmer chamber walls and atmosphere to cooler interior and exterior boxes. The acquisition of chamber environmental heat by the plant matter induces extra evapo-transpiration of commodity water, and returns the chamber atmosphere to near-saturation. Consequently, in-spite of the plant matter's reduced rate of metabolic heat production, the vacuum pump still evaporates the initial amount of water and the plant matter continues losing water at the initial rate.
Condensation in Cardboard
Sorption reactions generally occur over a short period of time, but if the adsorbed vapor begins to be incorporated into the structure of the sorbent a slowly occurring reaction known as absorption takes place. The difference between water adsorption and absorption is that adsorption is the attraction between the outer surface of a solid particle and water vapor, which leads to water condensation, whereas absorption is the uptake of the condensed water into the physical structure of the particle. The influence of a vacuum and atmospheric pressure on the internal mass transfer of water vapor in electrical-grade cellulosics, including cardboards used for high voltage insulation, has been studied. See KUTS, P. S., PIKUS, J. F. and KALININA, L. S. (1975) Coefficient of internal mass transfer in electrical-grade cellulosics under vacuum and under atmospheric pressure. J. Eng. Physics and Thermophysics 26 (4): 447-452 (“Kuts”). Kuts reported that moisture diffused through cardboard micro- and macropores 20 to 50-fold faster in a vacuum than at atmospheric pressure, and pure water vapor was adsorbed 50 to 150-fold more rapidly. In vacuum an initial steep rise in the rate of moisture adsorption on cardboard was followed by a rapid decrease in the rate of adsorption toward an end-point equilibrium, and eventually the condensed moisture was absorbed into the cardboard structure, lowering the cardboard's strength.
The adsorption and absorption of water which occurs in non-waxed cardboard boxes at a subatmospheric pressure differs from condensation on a heat exchanging surface in that the heat of condensation is released within the cardboard box and a major portion of said heat is transferred to plant matter present in the box. Condensation on a heat exchanging surface transfers the heat of condensation into the surface, allowing the heat to be harmlessly removed. Both types of condensation weaken cardboard boxes since water which condenses under the chamber roof drips back onto cardboard storage boxes. A Mylar® radiation reflecting slip sheet inserted between a non-waxed cardboard box's inner surface and plant matter in the box does not prevent the box from absorbing moisture and losing strength.
Measurements made during rose storages in a Vivafresh hypobaric warehouse (FIG. 4) revealed that water adsorption in non-waxed cardboard boxes rapidly released a large amount of latent heat. The water adsorption increased box weights by 12.7% during the first 14 days of storage, and most of this increase occurred within the initial 5 days. Released latent heat of water condensation transferred by radiation and convection to roses present in the boxes caused two-thirds of the 6.78% evaporative weight loss the roses experienced during a 5 week storage. Respiratory heat was responsible for the remainder of the weight loss, except for 0.24% caused by vacuum cooling the flowers from 4.1 to 2.5° C. during the initial pump-down. After pump-down was completed the release of latent heat from condensed water increased the cardboard's temperature, and its temperature remained warmer than the flowers' temperature until this trend reversed after 12.5 days (FIG. 4). By the 35th day water that had condensed in each cardboard box increased the cardboard's weight by 18%; in another test the cardboard increased in weight by 20.1% during 6 weeks, and in 4 weeks the high storage humidity caused an 18.6% cardboard weight gain regardless of whether a box was empty or filled with roses. By the 14th day each cardboard box (specific heat=0.44 cal/g·° C.) had condensed 195.2 g of water, releasing 594 calories of latent heat per gram of condensed water, a total of 115.2 kcal of heat. If this heat was not removed the cardboard's temperature would have increased by 213° C.! The box surface area transferring heat inward to the flowers by radiation and convection was 22.8-fold larger than the surface area radiating heat from one end of each box to the warehouse wall, and 11.4-fold larger than the area transferring heat by outward convection from both ends of the box into the warehouse's low-pressure air. Most of the released latent heat was radiated from the inner surface of the warmer box to the cooler flowers present in the box, while convection transferred a much smaller amount of heat from the box to the flowers since the convective heat transfer coefficient is reduced by 89% at 11.1 mm Hg. See Burg and Kosson, 1983; Equation 7. In another rose storage the latent (specific heat=0.87 kcal/kg·° C.) from 0.6° C. to 3.9° C. within a few days after the Vivafresh warehouse was initially evacuated, but when the warehouse was vented and re-evacuated on the 14th day no significant temperature rise occurred in the flowers, indicating that by then water condensation in the cardboard and the attendant release of latent heat had markedly slowed. Measurements of flower weight loss and cardboard weight gain, and visual examinations when the warehouse was vented and opened after 2, 3, 4, 5 and 6 weeks, indicated that water condensation beneath the warehouse roof became evident after 2 weeks. By then the humidity had become super-saturated and condensed moisture was dripping back onto the storage boxes and decreasing their strength. Subsequently a positive outward temperature gradient developed between the flowers and box, and heat began to be transferred by radiation and convection from the flowers to the cardboard, rather than in the reverse direction, while any heat still being generated by water condensing in the cardboard was transferred by radiation and convection from the cardboard to the vacuum chamber's wall and to the low-pressure storage atmosphere. Because the flowers now were the warmest objects in the storage space they no longer acquired heat from the cardboard or any other environmental source. Only metabolic heat was being removed by evaporative cooling, and since the production of respiratory heat decreases during storage (Hardenburg, 1986) the flower and box temperatures progressively declined (FIG. 4). This thermodynamic analysis was verified by comparing rose storage in non-waxed cardboard boxes vs. storage in equally sized plastic storage boxes that were unable to condense water. The floral weight loss was 5.0% during 15 days in non-waxed cardboard boxes and 1.57% in plastic boxes, including a 0.24% weight loss caused by vacuum-cooling during the initial pump-down. The entire commodity weight loss caused by water condensation in non-waxed cardboard was eliminated in plastic boxes, thereby reducing the water loss from plant matter to the amount needed to transfer respiratory heat. All subsequent commercial flower storages in the Vivafresh warehouse have been carried out in plastic boxes to minimize water loss. Mylar® typically is 69 to 80% effective in blocking radiation (Table 2), and a Mylar® liner situated between a non-waxed cardboard box's inner surface and roses present in the box was 70% effective in reducing the rose weight loss caused by released heat of water condensation. During hypobaric storage the cardboard ‘sleeves’ used commercially to protect roses during distribution at atmospheric pressure caused the same drying effect as non-waxed cardboard boxes. Roses protected with cardboard sleeves and stored in plastic boxes for 40 days in a laboratory vacuum chamber at 11 mm Hg, 2° C., lost 15% of their water and the weight of the cardboard sleeve's increased by 18%.
Controlling Air-Flow into and Through the Storage Space
In VacFresh intermodal containers an air mover, specifically a pneumatic air horn, mounted in an under-shelf duct accommodates the air-changes introduced by the entire range of pumping speeds and pressures recommended for different types of plant matter. See Burg, 2004; U.S. Pat. No. 4,685,305. Incoming air enters a concentric manifold chamber surrounding the pneumatic air horn's throat where jets located symmetrically around the concentric chamber are positioned to expand air into a reaction zone down-stream of the throat in which high-velocity air imparts its energy to slower moving air, accelerating the slower moving air's flow, in turn drawing more air through the throat into the reaction zone. The jets must discharge at supersonic velocity (turbulent flow) for the pneumatic air horn to operate at maximum efficiency. Said critical flow occurs when the upstream pressure is at least 1.9 times larger than the pressure downstream of the pneumatic air horns jets. The VacuFresh pneumatic air horn has been specially designed with jet orifices which satisfy this requirement when the vacuum pump operates at greater than 27% of its maximum capacity at 50 Hz.
Condensation Inside a Hypobaric Storage Space
When super-saturated air. mist or fog is evacuated from an LP storage space in which the pressure is controlled by a vacuum breaker operating at 25° C., the mist and fog vaporizes and the resultant air/water-vapor mixture warms to close to 25° C., elevating the pressure upstream of the vacuum pump. The pressure rise feeds-back into the storage space, and the vacuum breaker senses the increase in chamber pressure through an external register and responds by reducing the flow of atmospheric air into the storage space, thereby having the adverse effect of increasing the degree of super-saturation in the storage space. The pressure in a super-saturated LP system controlled by a vacuum breaker can only equilibrate when all excess water vapor condenses inside the storage space. Water vapor evapo-transpired from plant matter stored in plastic boxes does not continuously condense and liquid water accumulates during tests lasting 8 weeks when air saturated at the storage pressure and temperature by a mechanical humidifier continuously enters and flows through a leak-tight 13° C. laboratory chamber in which the pressure is controlled by an absolute vacuum regulator operating at 25° C. Under identical conditions large amounts of water vapor continuously condense and liquid water accumulates in the same storage space when the pressure is controlled with a vacuum breaker operating at 25° C.
Due to the low heat capacity of the air/water-vapor mixture present in an LP storage space, if super-saturated chamber air, mist or fog escapes in the air-change, this mixture's temperature rapidly increases from a storage temperature between 0 and 16° C., to a higher temperature as the mixture flows through a 25° C. heat transferring conduit (FIG. 1—K and FIG. 2—H) leading to a vacuum regulator operating at 25° C. (FIG. 1—H, FIG. 2—N, FIG. 3). Vaporization of the mist or fog, and gas and vapor expansion at the higher temperature increases the pressure of the air/water-vapor mixture while at the same time the mixture's RH decreases at the higher temperature. The elevated pressure feeds-back and raises the pressure in the storage space, and a vacuum regulator senses the rise in pressure through an external register connected to the storage space (FIG. 1—L and FIG. 3—J). The regulator responds by enhancing the pumping speed to off-set the pressure increase. Water vapor continues entering the storage space at the initial rate determined by the humidification system's wattage setting and a flow-regulating device (FIG. 1—D and FIG. 2—M), and the system equilibrates when the increased flow of air and moisture from the chamber to the vacuum regulator and vacuum pump decreases the chamber's RH from super-saturation to saturation, thereby preventing condensation from occurring. When 31.1 kg of mangoes were stored at 13° C. flowing one saturated incoming 15 mm Hg air-change per hour into a 170 liter LP chamber, a pressure regulator increased the pumping speed and rate of air-flow from the LP chamber to the vacuum pump, causing the same mass of air to exhaust from the vacuum pump 1.54-fold faster than said mass was entering the storage space (Table 3).
Previous Designs of Hypobaric Chambers
Attempts have been made to control the RH near saturation in Western and Chinese laboratory chambers in response to a humidity sensor, and a commercial hypobaric warehouse design envisioned a similar arrangement. See Li, W-X and ZHANG, M (2006) Effect of three-stage hypobaric storage on cell wall components, texture, and cell structure of green asparagus. J. Food Eng. 77 (1): 112-118 (“Li”); Li, W-X, ZHANG, M. and YU. H-Q (2004) Study on hypobaric storage of Asparagus officinalis. Wuxi University of Light Industry 23 (6): 38-42 (in Chinese) (“Li & Zhang”); TOLLE, W. E. (1969) Hypobaric storage of mature green tomatoes. USDA Agr. Research Rept. 842: pps. 1-9 (“Tolle 1969”); TOLLE, W. E. (1972) Hypobaric storage of fresh produce. Yearbook of United Fresh Fruit & Vegetable Association (July): pps. 27, 28, 33, 34, 36, 38, and 43 (“Tolle 1972”); ANON. 1974. A feasibility study of low pressure storage. Horticultural Science Dept. and School of Engineering, University of Guelph, Ontario, Canada, 46 pps (“Anon”). When Tolle stored strawberries and tomatoes in an LP apparatus without humidification “drying of the fruits increased at faster air-flow rates even though electro-sensors recorded the same high relative humidity irrespective of the airflow rate.” Lougheed reported that apples rapidly desiccated even though a humidity sensor indicated that rarified air in the chamber was nearly saturated. See LOUGHEED, E. C., MURR, D. P. and BERARD, L. (1978) Low-pressure storage of horticultural crops. HortScience 13(1): 21-27 (“Lougheed”). These anomalies were explained by tests carried out in a hypobaric intermodal container in which the humidity was measured and controlled by a bureau of standards chilled-mirror dew-point sensor. Whenever the relative humidity decreased below the sensor's highest reliable set-point a water boiler's electric immersion heater was energized to inject cold-steam into the low-pressure air-change entering the storage space. Without cargo present the system worked as envisioned, but after the intermodal container was loaded with 30,000 pounds of plant matter the humidification heater failed to energize because water vapor diffuses extremely rapidly across a very small vapor-pressure gradient at a low atmospheric pressure (FIG. 5A) and metabolic heat evaporated cellular water so rapidly through the plant matter's enormous surface area that the plant matter's temperature decreased slightly and approached the chamber air's dew-point temperature, causing heat to radiate and transfer by convection from the warmer container walls to the cooler plant matter. Respiratory heat and the acquired heat evaporated enough water from the plant matter to elevate the chamber RH above the humidity sensor's set-point, preventing the boiler's water heater from energizing, causing the plant matter to lose excessive water. A humidity sensor cannot reliably distinguish between water vapor generated from a humidification boiler in response to electrical heat vs. water vapor generated from plant matter in response to metabolic and environmental heat.
The mechanical humidification step described in U.S. Pat. Nos. 3,958,028, 4,061,483, 4,685,305, and US2010/0267144, continuously saturates the air in a hypobaric storage space, and the stored plant matter responds by increasing in temperature until a vapor pressure gradient is established that evapo-transpires sufficient water vapor to transfer most of the respiratory heat, thereby super-saturating the chamber atmosphere. See BURG, S. P. and ZHENG, Z. (2009) Experimental errors in laboratory hypobaric research and answer [A]. Chinese Assoc. Refrigeration [C] (“Burg, 2009”). For a perfect gas the buoyancy force (β) for natural convection is inversely related to the gas's temperature, but because in biological systems plant matter transfers heat by evapo-transpiration there is an additional buoyancy term due to the low molecular weight of the evaporated water vapor (MV=18) compared to air (MA=28.9). At atmospheric pressure and a 0 to 16° C. storage temperature, the buoyancy caused by water vapor's low molecular weight can be disregarded because water vapor represents an extremely small mole fraction of the air/water-vapor mixture present in the storage space, but water vapor's buoyancy is highly significant during hypobaric storage since water vapor constitutes upwards of 50% of the air/water-vapor mixture. See Burg and Kosson, 1983. Water vapor's high buoyancy rapidly lifts transpired water-vapor to the storage space's roof where heat is transferred from the air/water-vapor mixture into the roof since it serves as a heat exchanger in the refrigeration system. This heat transfer process caused condensation to occur beneath the roof in hypobaric warehouses and intermodal containers, and under the lids of glass vacuum desiccators and steel vacuum drums, on transparent viewing plates in steel laboratory vacuum chambers, and in leak-tight aluminum vacuum chambers when they were incubated in refrigerated laboratory cold-rooms. In commercial hypobaric systems the condensed water drips onto cardboard storage boxes and is absorbed into their structure, weakening the boxes. Bracing had to be installed and storage boxes covered with waterproof means to prevent box stacks from collapsing in hypobaric intermodal containers and warehouses. Microbial growth is promoted if condensed water wets the plant matter's surface, and surface water on plant matter is likely to be vacuum infiltrated into said matter's intercellular air spaces when the storage space is vented and opened, causing irreparable damage to the plant matter.
Flowing low-pressure saturated air-changes through a laboratory LP chamber failed to elevate the RH above 85% during 7 days when the chamber contained empty non-waxed cardboard boxes. See EP20100267144. The RH immediately increased to 99.5% when the boxes were removed. The non-waxed cardboard boxes were reducing the RH by adsorbing water. Water adsorption by cardboard is well known, and cardboard boxes often have been waxed to prevent water condensation (see Hardenburg) and a loss of box strength during high RH export shipments in refrigerated intermodal containers and storages in high humidity refrigerated warehouses. Because waxed cardboard boxes cannot be recycled they have been banned in Europe and replaced with water-resistant recyclable paperboard boxes (‘Solidboard’) capable of being hydrocooled without absorbing water and losing strength, and by plastic collapsible and returnable boxes. Cardboard boxes such as International Paper's Climaseries or Interstate Container's Greencoat™ are impregnated with wax alternatives making them recyclable, compostable, re-pulpable, water-proof or water-resistant, and suitable replacements for waxed cardboard boxes.
EP20100267144 suggests that specialized packing boxes unable to absorb water can be used as ‘an optional alternate feature’ to prevent non-waxed cardboard boxes from absorbing enough water to reduce the humidity and increase water loss from plant matter during LP storage, but when roses were stored in non-waxed cardboard boxes inside an LP warehouse through which two water-saturated air-changes per hour were flowing, the chamber humidity reached 96% immediately after pump-down was completed, within 36 hours the RH increased to 98.5%, and soon thereafter water began to condense under the chamber roof (FIG. 4). Cardboard's ability to lower the humidity was overwhelmed by water vapor transpired from the roses.
In 1979 Grumman Corp. and Armour & Co. were awarded the U.S. Food Technology Industrial Achievement award for developing the hypobaric transportation and storage system (see Mermelstein, N. H., 1979, Hypobaric transport and storage of fresh meats and produce earns IFT Food Technology Industrial Achievement Award. Food Tech. 33; 32-35; 38-40) (“Mermelstein”). Soon thereafter hypobaric research in the West came to an abrupt halt because peer-reviewed academic publications presented data ostensibly demonstrating that LP induced ‘stress’ ethylene production, failed to prevent ethylene action, accelerated diffusive water loss, and out-gassed flavor and aroma volatile organic compounds (VOCs) from vegetables and ripening fruits. Subsequent publications by BURG, S. P. and ZHENG, Z. (2007) Summary of hypobaric research in China and the West. Journal of Refrigeration 28 (2): 1-7 (in Chinese) (“Burg & Zheng, 2007”) and BURG, S. P. and ZHENG, X., 2009), Experimental errors in laboratory hypobaric research and answer. Chinese Assoc. Refrigeration (“Burg and Zheng 2009”) demonstrated that Western literature critical of hypobaric storage was based on experimental errors, including chamber leakage (FIG. 5B), humidification at atmospheric instead of a low pressure, cold spots on the LP chamber's surface due to non-precise temperature control, a poor understanding of autocatalytic and stress-ethylene production, an incorrect assumption that ethylene binding to its receptor is irreversible, a gross underestimate of the ethylene concentration needed to stimulate fruit ripening, failure to distinguish between low O2 and low-pressure effects, and allowing respiratory O2 consumption to inhibit flavor and aroma biosynthesis by creating an anaerobic condition in a static or nearly static LP system. More than 60 research papers describing favorable hypobaric storage results subsequently were published in China, and in 2010 the Science and Technology Committee of Shanghai issued a grant to develop a hypobaric warehouse, in 2011 the Science and Technology Committee of the People's Republic of China provided funding to build the first hypobaric storage unit for use on-board warships, and in 2013 the first Chinese hypobaric warehouse was sold.
U.S. Pat. No. 3,333,967, now reissue Pat. No. Re 28,995, to Burg and titled Method for Storing Fruit, the contents of which are herein incorporated by reference in their entirety, discloses a method for preserving mature but less than fully ripe fruits which produce ethylene and are ripened thereby, by use of hypobaric conditions of about 100 to 400 mm HgA pressure in a flowing stream of humidified, nearly water-saturated air.
U.S. Pat. Nos. 3,958,028 and 4,061,483, to Burg and titled Low Temperature Hypobaric Storage Of Metabolically Active Matter, the contents of which are herein incorporated by reference in their entirety, disclose a method of overcoming evaporative cooling and providing a constant high relative humidity in a hypobaric storage space. Incoming expanded atmospheric air is preconditioned to the pressure and temperature inside the storage space, and thereafter the air is contacted with a body of heated water to saturate the storage space atmosphere. A relatively broad spectrum of correlated conditions is disclosed that is operational in preserving metabolically active matter at pressures ranging from 4 to 400 mm HgA.
U.S. Pat. No. 4,685,305 to Burg and titled Hypobaric Storage Of Respiring Plant Matter Without Supplementary Humidification, the contents of which are herein incorporated by reference in their entirety, disclosed a method of preserving respiring plant matter without the step of humidifying the storage atmosphere by contacting the atmosphere with a supplementary body of heated water, characterized by storage at controlled and correlated conditions of temperature, atmospheric pressure, evacuation rate, air recirculation rate and air intake rate, and by the dependence of each set of correlated conditions upon the weight, respiration rate, and type of plant matter.
U.S. Pat. No. 4,655,048, to Burg and titled Hypobaric Storage Of Non-Respiring Animal Matter Without Supplementary Humidification, the contents of which are herein incorporated by reference in their entirety, discloses hypobaric storage of non-respiring animal matter without supplementary humidification.
Canadian Patent No. 997,532, to Burg and Burg and titled Prevention Of Microbial Growth By Treatment With Hypochlorous Acid Vapor, the contents of which are herein incorporated by reference in their entirety, discloses sodium or potassium salts of hypochlorite and carbonate that are added to humidification water of a hypobaric storage space at concentrations which cause CO2 present in incoming low-pressure air-change to continuously release a narrow range of hypochlorous acid vapor concentrations, thus killing molds and bacteria without injuring plant matter stored therein or leaving a harmful residue on the plant matter.
EP20100267144 to Burg, R. Bothel, and J. Bothel and titled Systems and Methods for Controlled Pervaporation in Horticultural Cellular Tissue, the contents of which are herein incorporated by reference in their entirety, discloses an apparatus and process providing the storage conditions as specified in U.S. Pat. No. 4,061,483 by use of a jacketed refrigeration system and secondary coolant to control a hypobaric intermodal container's temperature, as described in U.S. Pat. Nos. 4,061,483, 4,655,048, and 4,685,305, humidifying the atmosphere in the container by plant matter evapo-transpiration, as described in U.S. Pat. Nos. 4,655,048 and 4,685,305. In another embodiment, a steel hypobaric ‘Vivafresh’ warehouse is disclosed that is located inside an insulated space, wherein the temperature is independently controlled by a forced-air cooling system, as revealed in prior art describing a corten steel hypobaric storage space installed inside an independently controlled forced-air refrigerated enclosure. See Burg, S. P., 2004, Postharvest Physiology and Hypobaric Storage of Fresh Produce. CAB Int'l, Wallingford, Oxfordshire, UK, 654 pps. (“Burg 2004”). A computer software program adjusts the heater wattage in the Vivafresh warehouse's humidification boiler to saturate low-pressure incoming air-changes by contact with heated water, as described in U.S. Pat. Nos. 3,958,028 and 4,061,483, and the computer program also controls the storage pressure by means of three Clippard EVP series proportional solenoid control valves actuated in an algorithm responsive to an absolute pressure transducer.
Even when all known causes of excessive water loss have been eliminated hypobaric storages performed in commercial warehouses and during shipments carried out in LP intermodal containers have resulted in weight losses from fresh plant matter in excess of the amount predicted by laws describing heat exchange and mass transport at a low pressure, The weight loss problem needed to be better understood before the hypobaric method could be successfully developed into a commercial process for storing and transporting plant matter.